Click on the large map to zoom in on an area of interest. You can also use the +
and - buttons to zoom in and out, or the keyboard shortcuts Alt+Q to zoom out and Alt+S to zoom in

Pan the map using the links at the edges of the map, or the 'N' 'E' 'S' 'W' buttons.
You can also use the keyboard shortcuts Alt+W, Alt+D, Alt+X and Alt+A to pan the map

You can also pan the map by clicking the smaller overview map

Use the tabs under the map to change map style

The "Link to this Map" creates a nice accessible link to this map - which is tidier than many taken direct from the address bar.

Update 2015:

This map is starting to show its age, it was made over 10 years ago, with what is now old technology; which alas many modern browsers no longer fully support.
In particular the "open in new window"/tab does NOT function propelly, nor normal left-clicking on the map at thumbnail scale for some squares. And using other than 100% page zoom can cause issues.
For best results use Internet Explorer 10 or below (yes really!), or Firefox still seems functional.